[sdiy] Working with the ASM1 VCO.. weird MAT-02 behaviour...

[René Schmitz]

Hi Jean-Pierre,

Maybe you loose current through the protection diodes the MAT-02 and 
LM394 has between emitter and base. (Base of output transistor?)
Thats one of the differences to the 2n3904s.

Best,
  René

On 16.01.2020 17:48, Jean-Pierre Desrochers wrote:
> Hi list,
> 
> I'm working on the Elby-Design ASM-1 VCO
> 
> https://www.elby-designs.com/webtek/asm-1/vco/vco-asm1-cct.pdf
> 
> Im want to use it as a 1v/oct LFO (.05hz to 20hz)
> 
> I first mounted the circuit on my protoboard with success.
> 
> at first I used a matched pair of 2N3904 for Q1A & Q1B
> 
> instead of the MAT02 shown in the schematic.
> 
> I could reach very low frequencies (as low as .04hz).
> 
> But to do so I needed to replace R9(56k) for a 33k temporarily. Fine.
> 
> This morning I replaced both 2N3904 with a genuine MAT-02
> 
> and started the same frequency measurements as yesterday.
> 
> I bumped into something weird..
> 
> Here are the actual measurements I first did using 2x2N3904:
> 
> mVolt at 1k TEMPCO resistor vs Sawtooth period
> 
> 158.0mV => 0.050sec
> 
> 176.0mV => 0.100sec
> 
> 193.8mV => 0.200sec
> 
> 212.0mV => 0.400sec
> 
> 229.8mV => 0.800sec
> 
> 247.5mV => 1.6sec
> 
> 265.5mV => 3.2sec
> 
> 283.2mV => 6.4sec
> 
> 301.7mV => 12.8sec
> 
> 319.7mV => 25.6sec
> 
> (18mV steps, nice exponential response!)
> 
> Now here are the actual measurements with a MAT-02:
> 
> mVolt at 1k TEMPCO resistor vs Sawtooth period
> 
> 157.0mV => 0.050sec
> 
> 175.0mV => 0.100sec
> 
> 193.0mV => 0.200sec
> 
> 211.0mV => 0.400sec
> 
> 229.0mV => 0.800sec
> 
> 247.0mV => 1.6sec
> 
> 271.0mV => 3.2sec
> 
> **** Now trying to go further increasing the TEMPCO mVolts*
> 
> **** the oscillator starts backing up and re-increase its frequencies !!!*
> 
> **** I could not go further lowering the frequency. *
> 
> I tried 3 different MAT-02's and even LM394's
> 
> and they all behave the same.
> 
> Using different matched pairs of 2N3904 worked perfectly..
> 
> I suspect  MAT-02’s minimum base to emitter current reached (???)
> 
> What am-I missing here ??
> 
> By the way, all the IC's I'm using here are genuine one's
> 
> (TL082, MAT-02, CA3140, LM311).
> 
> JP
> 
> 
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